1. Field of the Invention
The present invention relates to detecting the presence of a reducing agent, such as ascorbic acid, in a test sample. More specifically, it relates to an analytical device and composition useful for determining the presence of the reducing agent in a liquid sample.
2. Discussion of the Prior Art
The analysis of different physico-chemical systems for the presence of reducing agents has been performed through the years by various methods, and utilizing varied chemistries. Some of these involve wet chemistry techniques, while others, for example C-STIX.RTM. reagent strips marketed by the Ames Company Division of Miles Laboratories, Inc., provide dip-and-read convenience.
Of the wet chemistry methods for analyzing the presence of reducing agents, several are of principal importance. These are the reduction of periodic acid, phosphomolybdic acid, Fehling solution, Tollens reagent, Nessler reagent, o-dinitrobenzene, p-nitrosodimethylaniline, selenous acid, and methylene blue. All of these wet chemistry techniques are discussed in Fritz Feigl, Spot Tests in Organic Analysis, Elsevier Publishing Company (1966) and H. R. Rosenberg, Chemistry and Physiology of the Vitamins, Interscience Publishers (1945).
Prior clinical determinations of ascorbic acid include titrations with iodine, 2,6-dichlorophenolindophenol, methylene blue, and by reaction with Folin's reagent, molybdenum-phosphotungstic acid, ferricyanide/ammonium molybdate, phosphomolybdic acid, uranyl acetate, vanadium, sulfanilic acid diazonium salt, sulfanilamide, selenous acid, gold trichloride, mercuric chloride, copper (II) sulfate/ammonium thiocyanate, iron (II) sulfate, iron (III) cyanide, hydrochloric acid (furfural test), cacotheline, permanganate and ferricyanide (Prussian blue method). All of these methods are described in H. R. Rosenberg, Chemistry and Physiology of the Vitamins, Interscience Publishers, New York (1945).
U.S. Pat. No. 3,771,964 describes a test composition and device for determining the presence of ascorbic acid in fluids such as urine. The composition comprises at least one phosphomolybdate salt and a nitrate salt. A dip-and-read device is prepared by incorporating the composition with a dry carrier material such as paper. The presence of ascorbic acid in the test sample will cause the device to turn blue, the color intensity being directly dependent on the concentration of ascorbic acid in the sample.
The present invention was discovered in the course of a research program aimed at finding a new reagent strip chemistry for determining ascorbic acid in urine. Specifically, a continuous response was sought over a range of ascorbic acid concentrations of about 40 to 200 mg%, this range including the ascorbate concentrations usually found in human urine.